The present invention relates to a surface-coated article and a method for the preparation thereof. More particularly, the invention relates to a shaped article provided with a hard and transparent ceramic-based coating film having high abrasion resistance and decorativeness without masking the beautiful appearance of the substrate surface as well as a method for the preparation of such a surface-coated article.
It is widely practiced that various kinds of metal articles such as personal ornamentals including watch cases and straps, necklaces, rings, spectacle frames, brooches, bracelets, earrings, medals and badges, orders and the like, tablewares, music instruments and so on are provided on the surface of a substrate body shaped of a metal or alloy such as stainless steels, nickel-based alloys, copper and copper-based alloys and the like with a metallic plating layer of gold, silver, platinum, rhodium, palladium, chromium and the like with an object to impart an impressive appearance of high-class goods.
A serious problem in these metal-made surface-plated articles is that the plating layer is sometimes subject to mechanical damages, falling and wearing during transportation and use when the article is contacted with a hard body or receives a mechanical shock to greatly decrease the commercial value and usefulness of the article.
Accordingly, it is also a practice widely undertaken in the prior art that the surface of personal ornamentals, musical instruments and the like is provided on the surface of the plating layer with a protective coating film formed by applying a clear and colorless lacquer in order to avoid the troubles mentioned above as far as possible. Such an organic coating film is not quite satisfactory in respect of the protecting effect due to the relatively poor abrasion resistance, corrosion resistance and weatherability.
It is also proposed that metal-plated surfaces are provided with a transparent inorganic coating layer of a ceramic material such as alumina, silica, zirconia, titanium dioxide and the like (see, for example, Japanese Patent Kokai Nos. 56-123366, 56-163266 and 61-165731. When such an inorganic transparent ceramic coating layer is formed on the surface of a metal or glass body by a conventional method such as vacuum vapor deposition, ion plating, chemical vapor deposition, sputtering and the like, some undesirable phenomena are sometimes unavoidable such as appearance of an interference color, discoloration and cloudiness in the coating layer to badly mask and affect the beautiful appearance of the substrate surface. Therefore, the applicability of these prior art methods is limited. In particular, the problem due to the above mentioned undesirable phenomena is serious on the surface of articles having a three-dimensionally complicated configuration such as watch cases and other personal ornamentals so that the above mentioned prior art methods are hardly applicable to these articles.
Apart from the above mentioned methods for forming transparent ceramic-based coating layers, ceramic bodies having transparency are known and obtained by a sintering process of a powdery mixture of aluminum oxide with 0.05 to 0.5% by weight of magnesium oxide (see, for example, U.S. Pat. No. 3.026,210). According to the process disclosed therein, aluminum sulfate and a magnesium salt are blended together in such a proportion that the weight ratio of Al.sub.2 O.sub.3 :MgO is 99.95:0.05 to 99.5:0.5 after calcination and sintering and the powdery mixture is calcined first at a temperature up to 1100.degree. C. and then at a temperature of 1200.degree. to 1300.degree. C. to prepare a calcined aluminum oxide powder containing magnesium oxide. The calcined aluminum oxide powder is then shaped and subjected to sintering by a heat treatment first at 800.degree. to 1200.degree. C. in air and then at about 1800.degree. C. for 6 to 10 hours in an atmosphere of hydrogen gas or in vacuum. The principle by which transparency is imparted to the thus sintered body is that formation of pores in the grains and abnormal grain growth are prevented by utilizing the phenomenon of grain-boundary precipitation of spinel MgAl.sub.2 O.sub.4.
This method for the preparation of a transparent alumina-based ceramic is of course not applicable to the formation of a ceramic-based thin protective coating layer on a metal-made and plated substrate article because the process disclosed so far gives only a shaped body of the ceramic material per se having bulkiness or a substantial thickness not to meet the definition of a thin coating film and, moreover, the temperature at which sintering of the ceramic body is performed is so high that not only the metal-made substrate body but also the layer of metal plating on the substrate surface have absolutely no possibility to withstand the temperature.